First you need to set ApplePay up: open Settings and tap Wallet & Apple Pay. Read the ApplePay overview, then tap Continue and tap Credit or Debit Card to add a card. Either use a card that’s already associated with your Apple account, or tap Add a Different Card, scan the card, then tap Continue. Once you’ve provided the card’s three-digit security code and agreed any additional terms and conditions, tap Continue to finish the process.
You can use ApplePay on any contactless terminal. Double-press the side button on the phone, select your card if you’ve more than one set up, authenticate with Face ID, then place the phone near the terminal just as you would a payment card.